Multiresolution Method of Spatially Filtering a Digital Image

1. A method of processing a digital image to improve tone scale, comprising the steps of: a) generating a multiresolution image representation of an original digital image including a plurality of base digital images and a plurality of residual digital images wherein: i) the base digital images are each of a different spatial resolution and are lower spatial resolution versions of the original digital image; ii) the residual digital images have different spatial resolution and include spatial detail relating to a different pass-band of spatial frequencies; b) applying a texture reducing spatial filter to the base digital images to produce texture reduced base digital images; and c) combining the texture reduced base digital images and the residual digital images to generate a texture reduced digital image; d) subtracting the texture reduced digital image from the digital image to produce a texture digital image; e) applying a compressive tone scale function to the texture reduced digital image to produce a tone scale adjusted digital image having a compressed tone scale in at least a portion of the image; and f) combining the texture digital image with the tone scale adjusted digital image to produce an enhanced digital image, whereby the contrast of the enhanced digital image is improved without compressing the contrast of the texture in the original digital image.

2. The method of claim 1 , wherein the step f) of combining includes the steps of scaling the texture digital image by a constant and adding the scaled texture digital image to the tone scale adjusted digital image.

3. The method of claim 1 , wherein the base digital images are generated using a two-dimensional Gaussian filter.

4. The method of claim 1 , wherein the base digital images are generated using a two orthogonal one-dimensional Gaussian filters.

5. The method of claim 1 , wherein the residual digital images are generated using a bi-linear interpolation filter.

6. The method of claim 1 , wherein the residual digital images are generated using a bi-cubic interpolation filter.

7. The method of claim 1 , wherein the base digital images are generated using two orthogonal one-dimensional Gaussian filters and the residual digital images are generated by applying a bi-linear interpolation filter to a base digital image to generate an interpolated digital image and subtracting the interpolated digital image from a next higher resolution base digital image.

8. The method of claim 1 , wherein wavelet filters are used to generate the multiresolution digital image representation of the original digital image.

9. The method of claim 1 , wherein the original digital image is a color digital image having a plurality of color channels and the tone scale function is applied to all of the color channels.

10. The method of claim 1 , wherein the original digital image is a color digital image having a luminance channel and a plurality of chrominance channels and the tone scale function is applied to the luminance channel.

11. The method of claim 1 , wherein the original digital image is a color digital image having a plurality of color channels and the tone scale function includes a component for each of the color channels.

12. The method of claim 1 , wherein the spatial filter is a sigma filter.

13. The method of claim 12 , wherein the original digital image is composed of pixels and the sigma filter comprises the steps of: identifying a pixel of interest and a local neighborhood of pixels located about the pixel of interest from the digital image to be filtered; calculating a difference pixel value for pixels in the local neighborhood of pixels based on the absolute value difference between the value of the pixel of interest and the individual values of pixels included in the local neighborhood of pixels; using the absolute difference pixel values to calculate a texture reduced pixel value; and replacing the value of the pixel of interest with the texture reduced pixel value.

14. The method of claim 13 , further including the steps of: comparing the absolute difference pixel values to a threshold value; and using only the values of pixels included in the local neighborhood of pixels for which the corresponding absolute difference pixel values are less than the threshold value to calculate the texture reduced pixel value.

15. The method of claim I, wherein the spatial filter is a masking filter.

16. The method of claim 15 , wherein the masking filter is an edge detecting masking filter.

17. The method of claim 15 , wherein the masking filter employs masking signal images from a plurality of base digital images to generate a texture reduced base digital image.

18. The method of claim 1 , wherein the tone scale function is derived from the original digital image.

19. The method of claim 16 , wherein the edge detecting masking filter includes the step of calculating gradient pixel values.

20. The method of claim 1 , wherein the step of generating the multiresolution image representation includes the steps of: a1) using two one-dimensional Gaussian filters and the original digital image to generate a first base digital image wherein the one-dimensional Gaussian filters are oriented in orthogonal directions to each another; a2) using a bi-linear interpolation filter and the first base digital image to generate an interpolated digital image; and a3) subtracting the interpolated digital image from the original digital image to generate a residual digital image.

21. The method of claim 1 , wherein the original digital image is derived from photographic film.

22. The method of claim 1 , wherein the original digital image is derived from a digital camera.

23. The method of claim 1 , wherein the spatial filter is applied to the base image(s) as they are being generated.

24. The method of claim 1 , wherein the spatial filter is applied to the base image(s) as they are being combined.

25. The method of claim 1 , further comprising the steps of applying a second texture reducing spatial filter to the residual digital images to produce texture reduced residual images, and using the texture reduced residual images to produce the texture reduced digital image.

26. A computer program product stored on a computer readable medium for performing the method of claim 1 .

27. Apparatus for processing a digital image to improve tone scale, comprising: a) means for generating a multiresolution image representation of an original digital image including a plurality of base digital images and a plurality of residual digital images wherein: i) the base digital images are each of a different spatial resolution and are lower spatial resolution versions of the original digital image; ii) the residual digital images have different spatial resolution and include spatial detail relating to a different pass-band of spatial frequencies; b) means for applying a texture reducing spatial filter to the base digital images to produce texture reduced base digital images; and c) means for combining the texture reduced base digital images and the residual digital images to generate a texture reduced digital image; d) means for subtracting the texture reduced digital image from the original digital image to produce a texture digital image; e) means for applying a compressive tone scale function to the texture reduced digital image to produce a tone scale adjusted digital image having a compressed tone scale in at least a portion of the image; and f) means for combining the texture digital image with the tone scale adjusted digital image to produce an enhanced digital image, whereby the contrast of the enhanced digital image is improved without compressing the contrast of the texture in the original digital image.

28. A method of processing a digital image to improve tone scale, comprising the steps of: a) generating a multiresolution image representation of the original digital image including a plurality of base digital images and a plurality of residual digital images wherein: i) the base digital images are each of a different spatial resolution and are lower spatial resolution versions of the original digital image; ii) the residual digital images have different spatial resolution and include spatial detail relating to a different pass-band of spatial frequencies; b) applying a texture reducing spatial filter to the base digital images to produce texture reduced base digital images; and c) combining the texture reduced base digital images and the residual digital images to generate a texture reduced digital image; d) subtracting the texture reduced digital image from the original digital image to produce a texture digital image; e) using the original digital image to generate a tone scale function and applying the tone scale function to the texture reduced digital image to produce a tone scale adjusted digital image; and f) combining the texture digital image with the tone scale adjusted digital image to produce an enhanced digital image, whereby the contrast of the enhanced digital image is improved without compressing the contrast of the texture in the original digital image.

29. The method of claim 28 , wherein the step of combining includes the steps of scaling the texture digital image by a constant and adding the scaled texture digital image to the tone scale adjusted digital image.

30. The method of claim 28 , wherein the base digital images are generated using a two-dimensional Gaussian filter.

31. The method of claim 28 , wherein the base digital images are generated using a two orthogonal one-dimensional Gaussian filters.

32. The method of claim 28 , wherein the residual digital images are generated using a bi-linear interpolation filter.

33. The method of claim 28 , wherein the residual digital images are generated using a bi-cubic interpolation filter.

34. The method of claim 28 , wherein the base digital images are generated using two orthogonal one-dimensional Gaussian filters and the residual digital images are generated by applying a bi-linear interpolation filter to a base digital image to generate an interpolated digital image and subtracting the interpolated digital image from a next higher resolution base digital image.

35. The method of claim 28 , wherein wavelet filters are used to generate the multiresolution digital image representation of the original digital image.

36. The method of claim 28 , wherein the original digital image is a color digital image having a plurality of color channels and the tone scale function is applied to all of the color channels.

37. The method of claim 28 , wherein the original digital image is a color digital image having a luminance channel and a plurality of chrominance channels and the tone scale function is applied to the luminance channel.

38. The method of claim 28 , wherein the original digital image is a color digital image having a plurality of color channels and the tone scale function includes a component for each of the color channels.

39. The method of claim 28 , wherein the spatial filter is a sigma filter.

40. The method of claim 39 , wherein the original digital image is composed of pixels and the sigma filter comprises the steps of: identifying a pixel of interest and a local neighborhood of pixels located about the pixel of interest from the digital image to be filtered; calculating a difference pixel value for pixels in the local neighborhood of pixels based on the absolute value difference between the value of the pixel of interest and the individual values of pixels included in the local neighborhood of pixels; using the absolute difference pixel values to calculate a texture reduced pixel value; and replacing the value of the pixel of interest with the texture reduced pixel value.

41. The method of claim 40 , further including the steps of: comparing the absolute difference pixel values to a threshold value; and using only the values of pixels included in the local neighborhood of pixels for which the corresponding absolute difference pixel values are less than the threshold value to calculate the texture reduced pixel value.

42. The method of claim 28 , wherein the spatial filter is a masking filter.

43. The method of claim 42 , wherein the masking filter is an edge detecting masking filter.

44. The method of claim 42 , wherein the masking filter employs masking signal images from a plurality of base digital images to generate a texture reduced base digital image.

45. The method of claim 28 , wherein the tone scale function is a compressive function such that the enhanced digital image has a reduced dynamic range relative to the original digital image.

46. The method of claim 43 , wherein the edge detecting masking filter includes the step of calculating gradient pixel values.

47. The method of claim 28 , wherein the step of generating the multiresolution image representation includes the steps of: a1) using two one-dimensional Gaussian filters and the original digital image to generate a first base digital image wherein the one-dimensional Gaussian filters are oriented in orthogonal directions to each another; a2) using a bi-linear interpolation filter and the first base digital image to generate an interpolated digital image; and a3) subtracting the interpolated digital image from the original digital image to generate a residual digital image.

48. The method of claim 28 , wherein the original digital image is derived from photographic film.

49. The method of claim 28 , wherein the original digital image is derived from a digital camera.

50. The method of claim 28 , wherein the spatial filter is applied to the base image(s) as they are being generated.

51. The method of claim 28 , wherein the spatial filter is applied to the base image(s) as they are being combined.

52. The method of claim 28 , further comprising the steps of applying a second texture reducing spatial filter to the residual digital images to produce texture reduced residual images, and using the texture reduced residual images to produce the texture reduced digital image.

53. A computer program product stored on a computer readable medium for performing the method of claim 28 .

54. Apparatus for processing a digital image to improve tone scale, comprising: a) means for generating a multiresolution image representation of an original digital image including a plurality of base digital images and a plurality of residual digital images wherein: i) the base digital images are each of a different spatial resolution and are lower spatial resolution versions of the original digital image; ii) the residual digital images have different spatial resolution and include spatial detail relating to a different pass-band of spatial frequencies; b) means for applying a texture reducing spatial filter to the base digital images to produce texture reduced base digital images; and c) means for combining the texture reduced base digital images and the residual digital images to generate a texture reduced digital image; d) means for subtracting the texture reduced digital image from the original digital image to produce a texture digital image; e) means for generating the tone scale function using the original digital image and means for applying the tone scale function to the texture reduced digital image to produce a tone scale adjusted digital image; and f) means for combining the texture digital image with the tone scale adjusted digital image to produce an enhanced digital image, whereby the contrast of the enhanced digital image is improved without compressing the contrast of the texture in the original digital image.